macOS: drop explicit -arch flags

[zeha]

Fixes #23.

Without -arch x86_64, the Apple toolchain automatically picks the target based upon the current runtime architecture.

Meaning:

• on Apple Silicon, running natively (= aarch64), the resulting binary will be for aarch64 (good!)
• on Apple Silicon, running under Rosetta, emulating x86_64, the resulting binary will be for x86_64 (good!)

Tests done:

• Builds on aarch64 macOS 11.1, Xcode 12.2 Apple clang version 12.0.0 (clang-1200.0.32.28). Both native and under Rosetta.
• Builds on x86_64 macOS 11.1, Xcode 12.2 Apple clang version 12.0.0 (clang-1200.0.32.27).
• My app works with a native (aarch64) Erlang 23.2.1 from Homebrew now.

[zeha]

@deadtrickster friendly ping - do you have objections/hints about this? thanks!

[deadtrickster]

Thank you for the PR and for the ping!

[bieniusa]

@deadtrickster I just ran into that issue - thanks for the fix!
Would it be possible to release a new version on hex?

[samgranieri]

@deadtrickster could you please release a new version of this on hex? Several engineers at my company are getting m1 Macs, and we use this library in every single one of our elixir apps. I'd rather not tell my colleagues to pin to a git ref if possible. Thanks for all the great work!